Coaxial attenuator for traveling wave devices



COAXIAL ATTENUATOR FOR TRAVELING WAVE DEVICES Filed 0012. 31, 1966 P 969A. H. DOWNING E AL 5 Sheets-Sheet l flaszS/l ve Maia/ml uvvmrans ARTHURH. DOWN/N6 55m? JAN/S/ ATTORNEY Sept. 2, 1969 COAXIAL ATTENUATOR FORTRAVELING WAVE DEVICES I Filed Oct; 31, 1966 A. H. DOWNING ET L 5Sheets-Sheet 2 I fisA z ATTORNEY p 969 A DOWNING ET AL 3,465,198

COAXIAL ATTENUATOR FOR TRAVELING WAVE DEVICES Filed Oct. 31, 1966 5Sheets-Sheet 5 l/VVE/VTO/PS ARTHUR H. DOWN/N5 75/? JAIN/5 5y ATTORNEYUnited States Patent M 3,465,198 COAXIAL ATTENUATOR FOR TRAVELING WAVEDEVICES Arthur H. Downing, Woburn, and Peter Janis, Auburndale, Mass.,assignors to Raytheon Company, Lexington, Mass., a corporation ofDelaware Filed Oct. 31, 1966, Ser. No. 590,901 Int. Cl. H01j 25/34 U.S.Cl. 315--3.5 7 Claims ABSTRACT OF THE DISCLOSURE The present inventionrelates generally to traveling wave devices having a slow wavetransmission circuit and more particularly to a coaxial termination fordissipation and absorption of electromagnetic energy propagated in suchcircuits. In addition impedance matching means are provided for thereduction of anomalous oscillations arising from either forward orbackground wave interaction between the beam of electrons and the energypropagating on the slow wave circuit.

Traveling wave devices of the type under consideration may be employedas amplifiers or oscillators and such devices conventionally include aperiodic slow wave transmission structure such as an interdigital delayline disposed within an evacuated envelope. A sole electrode of either alinear or circular configuration is spaced from the slow wave circuit ina coplanar manner and defines therewith the boundaries of an interactionspace. Means for the generation of an electron beam is disposed at oneend thereof and means for the collection of electrons is located at theopposing end. Suitable electrical biasing potentials are establishedbetween the sole electrode and slow wave circuit and external magnetsprovide for the establishment of a magnetic field transverse to theelectric field. Hence, such devices 'are often referred to as being ofthe crossed-field type. Under the combined influence of the electric andmagnetic fields electromagnetic wave energy propagating along the slowwave circuit interacts with the electron beam traversing the interactionspace in an energy exchanging manner. Suitable adjustment of the fieldparameters as well as beam trajectory will result in forward or backwardwave interaction with oscillation and/or amplification of radiofrequency energy.

An exemplary slow wave circuit of the interdigital finger type isdisclosed in US. Patent No. 2,925,518. issued Feb. 16, 1960 to John M.Osepchuk and assigned to the assignee of the present invention.Specifically, the finger elements comprise a first portion extendingperpend-icular to a common base, or as sometimes referred to, back wall,and attached thereto. A second portion extends perpendicular to thefirst portion and parallel to the common base, while a third portionextends perpendicular to the second portion in a direction extendingback toward the common base and defining a free space at the extremitythereof. Such interdigital delay line elements are referred to asI-fingers and as a result of this configuration the over-allelectromagnetic wave energy path has been lengthened while maintainingthe 3,465,198 Patented Sept. 2, 1969 over-all magnetic field gapconstant to result in much lower frequency of operation and higher powerhandling capabilities.

All traveling wave devices of the crossed-field type are desirablyoperated in a nonreentrant manner to achieve power output stability aswell as eliminate spurious or anomalous oscillations during operation.For the purposes of this description the term nonreentrant refers to therequirement that the electron beam traverse the interaction space onlyonce in an energy exchanging relationship and be as completely expendedas possible after the first traverse. In addition, the term is utilizedin referring to the slow Wave transmission circuit to indicate thatelectromagnetic wave energy propagated thereon is permitted to traversethe circuit in only one direction without any reflection of such energyby any discontinuities or (mismatching of impedances. In prior artdevices preselected portions of the delay line structure have functionedas electromagnetic energy absorption means and still be required tomaintain the electrical characteristics of the over-all slow wavestructure at a predetermined value. Energy absorbing coatings ofmaterials such as iron or other suitable lossy materials have beenconventionally disposed on a plurality of the delay line elements nearthe terminal portion of the interaction space. Such coatings of iron orother resistive materials have been difficult to apply uniformly to thedelay line elements with resultant poor adherence. Further, suchcoatings which are inherently of a carbon composition create problemswhen the devices are evacuated. The introduction of coating materialssuch as iron also varies the magnetic field parameters which must becarefully controlled for suitable operation.

At the high power levels of operation in a large number of such devicesthe heat resulting from the absorption of the electromagnetic waveenergy, particularly when continuous wave operation is anticipated, hasoftentimes resulted in deformation of the interdigital delay lineelements thereby introducing limitation in usefulness. Rather lengthyexternally coupled attenuating structures have therefore evolved in theart in an attempt to further extend the range of the power handlingcapabilities of subject devices, particularly those operating in thelower end of the microwave frequency spectrum. It has been noted,however, that such elongated or lumped lossy structures for theabsorption and attenuation of the undesired electromagnetic wave energyand electron beam residual electrons introduces many variables in thecapacitance and/or inductance of the over-all slow wave delay linedesign parameters, particularly when attenuators of many wavelengths inover-all dimension are appended to the slow wave circuit structure.

In the present invention a new and novel type attenuator is disclosedhaving one member directly coupled to the terminal end of a delay lineand defining with a coaxial member transformation means for matchingimpedances. Further, the over-all length of the coaxial attenuationarrangement is a small fraction of the operating wavelength to therebyprovide an essentially pure resistive component in the slow wavecircuit. Additionally, selected materials are disclosed of a whollyresistive characteristic to further enhance the attenuation capabilitiesin the absorption of heat and rapid conduction of said heat away fromthe critical region. In illustrative embodiments of the presentinvention bulk as well as thin film attenuation means are disclosedhaving high heat handling capabilities to enable generation of higheroutput power. The average coaxial impedances. of the attenuatorstructure are suitably matched by transformation means to the impedancecharacteristics of the applicable slow wave transmission circuit. Theinvention is equally applicable to many delay line configurationsincluding the J-shaped elements in lower frequency devices. Throughoutthe description it will be noted that the illustrative configurations ofthe invention are provided having a very short over-all dimension.External coolant means communicating with the coaxial attenuator willalso be described to further aid in the rapid conduction of the heatgenerated by the attenuation of the undesirable electromagnetic waveenergy.

A primary object of the present invention is the provision of animproved resistor coaxial attenuator for traveling wave devices having aslow wave conducting structure.

Another object of the present invention is the provision of a coaxialattenuator for crossed-field traveling Wave devices having incorporatedtherein means for the matching of the impedances of the coaxialstructure to the slow wave transmission circuit.

Still another object of the present invention is the provision ofelectromagnetic energy absorbing means in a crossed-field device havingan interdigital delay line structure to thereby provide for moreefficient nonreentrant operation with regard to the electromagnetic Waveconducting path as well as the electron beam path.

A still further object of the present invention is the provision of acoaxial attenuator for crossed-field traveling wave devices wherein theenergy absorbing means comprise a relatively thin layer of a resistivematerial having a high microwave energy absorption capability depositedon a bulk material having a rapid heat conduction capability to permitthe operation of such devices at higher output powers for bothintermittent and continuous operation.

Further objects, features and advantages of the present invention willbe readily apparent after consideration of the following detaileddescription together with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view of an illustrativeembodiment of the invention;

FIG. 2 is a detailed cross-sectional view along the line 2-2 in FIG. 1;

FIG. 3 is a fragmentary cross-sectional view of another embodiment ofthe invention;

FIG. 4 is an enlarged fragmentary view partially in cross-section of anauxiliary coolant circulation means which may be utilized in conjunctionwith the embodiment of the invention;

FIG. 5 is a fragmentary cross-sectional view of still another embodimentof the invention;

FIG. 6 is a graph of the standing wave ratio in relation to the ratio ofthe diameter of the inner and outer members of the coaxial attenuator;

FIG. 7 is a fragmentary view of a portion of the slow wave structure inFIG. 2 with an additional modification;

FIG. 8 is a partial cross-sectional view of the utilization of theembodiment of the invention in a linear interdigital delay line;

FIG. 9 is a fragmentary cross-sectional view of another alternativeembodiment of the invention; and

FIG. 10 is a similar view illustrative of a modification in theembodiment shown in FIG. 9.

Referring to the drawings, FIGS. 1 and 2 illustrate a backward wavecrossed-field device 10 incorporating a slow wave propagating structure14 of the interdigital delay line type with the individual delay lineelements being of the I-shaped configuration disclosed in theaforementioned United States patent. Sole electrode 12 is disposedconcentrically with respect to the delay line 14 and is normallymaintained at a negative potential with respect thereto. An inputelectrical lead assembly 16, electron gun assembly 18 including anindirectly heated cathode 19, magnetic field producing means 20 andoutput coupling means 22 complete the major subassemblies of theover-all embodiment. The arcuate interdigital delay line 14 comprises aplurality of interdigital elements secured to cylindrical base member 24which together with the oppositely disposed cover plates 26 and 28hermetically sealed thereto form the walls of evacuated envelope of theover-all embodiment 10.

The sole electrode 12 comprises a cylindrical member of an electricallyconductive material and includes a web portion 30 bounded by an arcuateportion 31 defining a channel 32 for the purpose of confining theelectron beam within the interaction space 33 defined by the channelwall surfaces and the delay line assembly 14. One end of a hollowsupporting member 34 is inserted within a tubular member 35 which is inturn secured to the sole electrode web portion 30. Member 34 in additionto supporting the sole electrode forms a portion of the electrical leadassembl 16 and permits the introduction of external circuit connectingleads to appropriate electrodes within the over-all device. A slottedsection 36 is defined within the sole electrode and the electron gunassembly 18 is disposed therein. A mounting plate 37 provides means forattachment to the web portion 30. The gun assembly includes a cathode,heater, grid and accelerating electrodes of the known configuration andthe details have not been enumerated herein for the sake of clarity.

The input electrical lead assembly 16 comprises a sleeve member 38secured to cover plate 26 together with a dielectric sealing member 39joined at its outer end to a second electrical conductive sleeve member40. A terminal glass bead seal 41 supports the electrical leads inspaced relationship and hermetically seal the tube envelope. Electricalenergy from appropriate sources is supplied to the gun assemblyelectrodes by way of electrical lead wires 42, 43, 44 and 45 whichextend through the glass bead 41.

The required electric field between the slow wave structure 14 and soleelectrode 12 is supplied by means of a unidirectional voltage suppliedtherebetween, such as, for example, a battery 47. The sole electrode 12is preferably biased negatively with respect to the cathode 19 by meansof a source 48 connected between the cathode lead 43 and the soleelectrode supporting member 34 by way of sleeve member 40. Similarly,the line 14 will be maintained at a positive potential relative to boththe sole electrode 12 and cathode 19 by the source of unidirectionalvoltage 47 connected between the sleeve 38 and cathode lead 43 with thesleeve member 38 being connected through base member 24 to the delayline. Lead 44 may for example be utilized to supply a positive potentialrelative to the cathode to the accelerating electrode by means of asource 50 connected between leads 43 and 44. The remaining electricallead member 45 may be connected by way of a terminal 51 to anappropriate energy source for controlling the magnitude of the electronbeam current in the oscillator 10.

Output coupling means 22 comprise a coaxial transmission line having anouter conductor 52 and an inner conductor 53 with its inner end securedillustratively to delay line element 54 adjacent to the electron gunassembly 18. In this manner of output coupling the cross-field travelingwave device will operate in the backward wave mode of oscillation. Acollector electrode 55 is disposed at the opposing end of theinteraction space 33 and provides for the interception of the residualelectrons in the beam after the first traversal of the interactionspace. Collector electrode 55 may be tapered and is joined to the basemember 24.

A uniform magnetic field transverse to the direction of the electronbeam and electric fields is provided by the magnet assembly 20 includingpole piece members 56 and 57 with the major interaction components beingdisposed in the magnetic gap defined therebetween. Permanent magnetmembers or any other suitable electromagnetic means will contact thepole piece members.

The coaxial attenuation arrangement in accordance with the principles ofthe invention is incorporated in the structure designated generally bythe numeral 60.

This arrangement provides for the termination of the electromagneticenergy propagating path by a short-circuited coaxial transmission linehaving a resistance element disposed between the inner conductor memberand the short circuit end with the length of the attenuator being asmall fraction of a Wavelength at the frequency of operation. Thecoaxial termination and attenuator 60 includes a center conductor member61 which is joined at one end to the delay line finger element 62disposed at the end of the interaction path adjacent to the collectorelectrode 55. Inner conductor member 61 may be fabricated of anymetallic material having a coeflicient of expansion compatible with theelectromagnetic energy absorbing attenuating material and in anillustrative embodiment molybdenum was selected as having the requisitecharacteristics. An ideal resistive material for attenuation of theelectromagnetic Wave energy was found to be silicon carbide. Cylindricalmember 63 composed of this material is disposed in contact With theinner conductor member 61 and the short circuit end 64 of the outerconductor member 65 which is mounted and appended to the envelope wallmember 24. Outer conductor member 65 is preferably of a highconductivity metal such as copper which is similar to the conductivematerial commonly employed in traveling wave tube envelope walls.

In the electrical considerations for the coaxial attenuator structure itis of paramount importance that the impedance of the coaxial line beclosely matched to that of the substantially parallel plate delay linein order to eliminate reflections of electromagnetic wave energy whichwill result in undesirable spurious oscillations. In the termination ofelectromagnetic energy transmission circuits with a resistor element thecharacteristic impedance of a line in ohms (Z when the value of R forthe series resistance of the line is zero is derived from the classicalequation log where D is the inside diameter of the outer conductor and dis the diameter of the resistor. In oscillator devices of the typeillustrated in FIGS. 1 and 2 when operated in a microwave radiofrequency range of 900 to 1500 megacycles the characteristic impedancesof the delay line propagating structure is approximately 150 to 180ohms. The coaxial attenuator termination therefore to be provided with amatching impedance requires the ratio of the diameters of the inner andouter conductors to be such that either the outer conductor will beunusually large or the center conductor would be unusually small. Thecombined configuration would destroy the desired circuit parameters andwould not dissipate the heat generated at the high power levels desired.A step or impedance matching transformation means is therefore desirableto couple the high delay line impedances to a more useful coaxialimpedance, illustratively 50 ohms. Such transformation means areprovided by a stepped portion 66 provided in the inner walls of outerconductor 65 concentrically disposed about the inner conductor member61. The lumped attenuator material rapidly absorbs the electromagneticenergy and the heat generated is rapidly dissipated by the thermallyconductive copper material of the outer conductor walls at the shortcircuit end.

To further increase the heat dissipation capability the inner walls ofouter conductor member 65 may be gradually tapered adjacent the terminalend 64 as at 67 to provide a larger bulk of conductive material as shownin FIG. 3. In this as well as subsequent illustrations similar orcorresponding parts have been designated by the same reference numeralsas those shown in FIGS. 1 and 2.

In FIG. 4 the thermal and power handling capability of the attenuatorarrangement 60 incorporating resistor element 63 is enhanced by externalcoolant circulation means designated generally 70 which will now bedescribed. In this embodiment a coolant jacket is defined by opposingupper and lower wall members 71 and 72 together with a lateral wallmember 73 joined to the outer peripheral Wall of the envelope member 24.Inlet port 74 and outlet port 75 are provided in wall 73 for ingress andegress of the selected coolant which is circulated by conventional means(not shown) in contact with the outer walls of the outer conductormember of the coaxial attenuator 65. By means of any of the knowncoolants the traveling wave device may be utilized at even higher powerlevels than those attainable without conduction cooling. To furtherenhance the efficiency of the thermal conduction the outer walls ofmember 65 are provided with fins 76 circumferentially disposed aboutthis member. The increased radiative surfaces exposed to the coolantwill permit higher power levels of operation.

In FIG. 5 still another configuration of a lumped coaxial attenuatorconfiguration is shown. The delay line finger element 62 is terminatedby inner conductor member 61 which is in turn joined to a bulk resistiveattenuator structure 80 of a silicon' carbide composition having taperedwalls 81. The outer conductor member 82 is provided with similar taperedwalls 83. The resistor 80 is terminated in wall member 84 which isjoined to the outer conductor 82 by any conventional means. In thisembodiment the resistor 80 is firmly bonded as by brazing to a recessedportion of member 84 to provide for good heat conductivity. The taperedor flared configuration may illustratively be from a diameter at theapex of resistor 80 of .130 inch to a value of .350 inch at the base ofthe resistor.

When resistors are utilized for terminating an electromagnetic waveenergy propagating transmission line the principal cause of standingwaves is the presence of reactance. In a paper entitled Radio-FrequencyResistors As Uniform Transmission Lines by D. R. Crosby and C. H.Penneypacker, Proceedings of the I.R.E., February 1946, pp. 62P-66P, atheoretical analysis of the classical transmission-line equationincludes a plot of curves shOW- ing how standing-wave ratio varies withfrequency. FIG. 6 of the drawings is now referred to with the standingwave ratio plotted along the vertical coordinate and a ratio R/Z plottedalong the horizontal coordinate. The value of R/Z is determined by theratio of the resistor diameter to the diameter of the transmission linesurrounding the resistor. From this group of curves it will be notedthat the standing wave ratio approaches unity at a value ofapproximately R/Z 3 which is approximately 1.73. From the referencedpaper it is ascertained that the term R/Z is independent of frequency.In the determination of the frequency characteristics of the resistorthen another term U). which is proportional to frequency may becalculated from the equation:

l fmc 11,800

where l is the length of the resistor in inches, A is the free spaceWavelength in inches and fmc is the operating frequency in megacycles.It is desirable to operate in a frequency range where the standing ratiowill be at the highest value approaching unity. Curve 86 or the value ofl/)\ equal to 0.05 indicates that the standing Wave ratio will be 0.97at the optimum value of R/z If this value of l/ is put in the foregoingequation it will be noted that a resistor 1 inch long would have goodcharacteristics up to 600 megacycles. Since most high power travelingwave tubes operate up to from 1200 to 1500 megacycles a resistor 63 inFIGS. 1 through 4 would have an approximate length of .400 to .500 inch.Further, the optimum diameter proportions would be fixed at .130 inchfor the resistor element and approximately .260 inch for the innerdiameter of the surrounding conductor to result in a value approximatingthe /3 to achieve the optimum standing wave ratio. It will also beevident that as the length of the resistor is increased, for example to12 inches, the coaxial termination would result in maximum operation atfrequencies of only up to 50 megacycles.

Another modification which may be practiced in the present invention isillustrated in FIG. 7. In this embodiment the coaxial attenuatortermination 90 in a device similar to that illustrated in FIGS. 1 and 2is directly coupled to the next to the last delay line finger element 91and the coupling point is spaced approximately onequarter of anelectrical wavelength away from the terminal portion of the over-alldelay line structure. Such one-quarter wavelength spacing provides aradio frequency choke arrangement which coupled with the impedancematching transformer means 66 will further aid in the reduction of theundesired reflections of electromagnetic wave energy.

Referring next to FIG. 8, this configuration is a linear delay lineincluding two parallel bars 93 and 94 which support opposing series ofstraight interdigital finger members 95 and 96 to thereby define theserpentine electromagnetic energy path. The coaxial attenuator 97 isprovided with an inner conductor 98, energy absorbing resistor 99 andouter conductor 100 with the impedance matching transformer meansdefined by the wall 101. The numerous other previously describedmodifications may be employed for the removal of the heat generated inthe walls of the attenuator, such as a surrounding coolant jacket.

Referring now to FIG. 9, a modification of a bulk type attenuatorstructure having tapered walls similar to that shown in FIG. will now bedescribed. A lossy ceramic member 102, for example alumina, is providedwith tapered walls 103. The inner conductor 104 joined to theappropriate delay line element is disposed in contact with the apex ofthe tapered dielectric body. The bonding of the lossy material to themetallic inner conductor 104 to withstand the wide temperature range ofoperation will be aided by a serrated conductive sealing member 105,illustratively of copper, united by known brazing techniques to the bulkmaterial. One of the inherent qualities of electromagnetic wave energyis the relatively shallow depth of penetration of the current whichenables relatively thin films to be employed in the provision of aresistance in the transmission line. A resistive layer 106 deposited onthe tapered walls of the bulk lossy material 102 may be for examplecarbon having a resistance value of approximately 50 ohms. Such a thinfilm layer 106 where a high density alumina material is employed may bedeposited by exposing the ceramic material to a heated atmosphere ofbenzene. The deposition of the thin resistive film may be accuratelycontrolled to achieve any desired values of resistance. Another sealingmember 107 of the serrated configuration and similar material unites thehigh density bulk member 102 to the short circuit end 108 of a highlyconductive metal for the rapid removal of the heat absorbed in theresistance layer and ceramic member.

FIG. illustrates a modification in the structure shown in FIG. 9 andwhere applicable similar numerals will indicate similar structure. Ahollow lossy dielectric member 109 which may be provided in a tapered orcircular configuration is mounted concentrically on a relatively largeconductive body 110 of a high thermal conductivity material such ascopper. The short circuit end of the body 110 is indicated at 111. Theheat generated in the lossy material by the energy absorbed in the thinfilm layer 112 deposited on the outer walls of the hollow ceramic body109 will be rapidly conducted away from the terminal end of the slowwave structure in communication with the inner conductor 104.

There is thus disclosed a novel coaxial attenuator of the bulk or thinfilm type for absorption of electromagnetic energy propagating in a slowwave transmission circuit. Reflections of such energy have beensubstantially reduced and through the provision of transformer meansmatching the impedances of the coaxial attenuator structure to theimpedances of the slow wave circuit no redesign of the latter isrequired. The primary advantage of the present invention resides in thefact that through the provision of a resistor element at the terminalend of the energy path the overall length of the coaxial attenuatorarrangement will be only a fractional part of the operating wavelengthof a traveling wave device. Hence, structures as short as one inch arecapable of dissipating extremely high temperatures and many watts of DCelectrical energy generated in the terminal components. Such disclosedstructure has enabled traveling wave devices to be operated efiicientlyin continuous wave operation in the generation of high output power,particularly at the lower frequency band of the electromagnetic waveenergy spectrum. The disadvantages of the prior art plating techniquesrequiring closer delay line element spacings as well as smallercross-sectional areas for optimizing of the electrical characteristicsof the slow wave transmission circuit when the attenuating material isincorporated directly in the finger elements have been substatniallyeliminated. Numerous other materials such as pyrographite film layercoatings as well as beryllium oxide ceramic materials may also beutilized in the practice of the invention to further enhance theabsorption and thermal conductivity capabilities.

The invention including the numerous modifications and alterationsevident to those skilled in the art is accordingly to be interpretedbroadly in accordance wth the scope and spirit as set forth and definedin the appended claims.

What is claimed is:

1. In combination:

a metallic envelope;

an interdigital delay line within said envelope for propagatingelectromagnetic energy at a predetermined frequency;

means for generating and directing a beam of electrons along a pathadjacent to said delay line;

and means for terminating said delay line and absorbing electromagneticenergy comprising a coaxial attenuator extending laterally outside saidenvelope and having an inner conductive member connected to a discretepoint of said delay line;

an outer conductive member concentrically disposed about said innerconductor and having a short-circuited end wall;

said outer conductive member further defining wall structure forsubstantially matching the impedances of said delay line and coaxialattenuator;

a member of a highly resistive material disposed coaxially with saidinner conductor and contacting said short-circuited end wall;

said coaxial attenuator having a length of a fractional part of awavelength of said predetermined frequency;

and means for circulation of a coolant along the outer walls of saidouter conductor concentrically disposed about said envelope.

2. In combination:

a metallic envelope;

an interdigital delay line within said envelope for propagatingelectromagnetic energy at a predetermined frequency;

means for generating and directing a beam of electrons along a pathadjacent to said delay line;

and means for terminating said delay line and absorbing electromagneticenergy comprising a coaxial attenuator having an inner conductive memberconnected to a discrete point of said delay line;

an outer conductive member concentrically disposed about said innerconductive member and having a short-circuited end wall;

said outer conductive member further defining wall structure forsubstantially matching the impedances of said delay line and coaxialattenuator;

a member of a highly resistive material disposed coaxially between saidinner conductive member and said short-circuited end wall, the diameterof said resistive member being substantially equal to the diameter ofsaid inner conductive member and the ratio of the resistance of saidresistive member to the line impedance determined by the ratio of thediameters of said resistive member and said outer conductive memberbeing substantially equal to the square root of 3;

said resistive member further having an over-all length of a fractionalpart of a wavelength of said predetermined frequency.

3. In a traveling wave device having a slow wave transmission structurefor propagating electromagnetic energy at a predetermined frequency,means for absorbing said energy directly coupled to said slow wavestructure comprising:

a coaxial attenuator including an inner metallic member connected at oneend to a discrete point of said slow wave structure;

an outer cylindrical metallic member concentrically disposed about saidinner member and terminating in a short-circuited end wall;

a member having a high resistance value to electrical energy joined atone end to said inner member and at the opposing end to said end wall;

said resistive member having an over-all length of a fractional part ofa wavelength of said frequency and the ratio of the resistance of saidmember to the line impedance determined by the ratio of the diameters ofsaid resistive member and said outer member being substantially equal tothe square root of 3.

4. A- traveling wave device according to claim 3 wherein said resistivemember has tapered walls and the inner walls of said outer conductor aresimilarly tapered and coextensive with said member.

5. A traveling wave device according to claim 3 wherein said resistivemember consists of silicon carbide.

6. A traveling wave device according to claim 3 wherein said resistivemember comprises a body of a lossy dielectric material having asubstantially thin layer of a high electrical resistance materialdeposited on the outer peripheral Walls.

7. A traveling wave device according to claim 3 wherein said resistivemember comprises a hollow cylindrical body of a lossy dielectricmaterial mounted on a highly conductive metallic member and asubstantially thin layer of a high electrical resistance materialdeposited on the outer peripheral walls of said hollow body.

References Cited UNITED STATES PATENTS 2,839,730 6/1958 Rosenberg 333222,863,092 12/1958 Dench 3153.5 X 2,922,918 1/1960 Wasserman 3 l53.5

HERMAN KARL SAALBACH, Primary Examiner PAUL L. GENSLER, AssistantExaminer US. Cl. X.R. 31539.3; 333-81

